A hypodermic needle contains a hollow metal tube having a lumen opening extending in an nominal diagonal line across the distal end of the tube. The opening comes to a tapered point or lancet at the tip of the needle and the walls of the tube present a beveled face. The tapered, diagonal opening and beveled face facilitate skin puncture and delivery of the therapeutic agent by the tip of the needle.
In most hypodermic needle manufacturing operations, one or more grinding steps are used to shape the needle tip. Needle grinding methods include processes for grinding single needles and, more often, magazines or assemblies containing a plurality of needles in a parallel array. Traverse grinding and/or oscillating plunge grinding processes are used most frequently. Automated operations for continuous production of needles have been described. See, for example, U.S. Pat. Nos. 4,104,833, 4,384,942 and 5,575,780, which are hereby incorporated by reference. One persistent problem encountered in needle grinding operations is the formation of burrs and metal fines on the needle tips.
The presence of metal burrs, fines or slivers on the needle tips may create pain or other adverse consequences during use. Grinding processes used in needle manufacturing commonly produce burrs and fines. To avoid burr formation and to create the sharp, precise beveled face of the needles, typical grinding wheels used commercially in needle finishing must be dressed at frequent intervals. Frequent dressing causes a loss of grinding wheel life and increases the cost of the manufacturing operation. In some operations, the needles also are polished, cleaned with electrolytic treatment (see, e.g., GB-B-1,170,848) or otherwise further processed after grinding to insure removal of such debris. Such additional processing steps represent an undesirable manufacturing expense and complication.
In the alternative, burrs have been eliminated or substantially reduced by using a very soft bond (e.g., an alkyd polyester resin bond, or a shellac bond), but such wheels have an unacceptably short grinding life and are not commercially viable solutions to the problem of burrs. Fillers having a lamellar structure, such as graphite, molybdenum disulfide and hexagonal boron nitride, are suggested in FR-A-2,330,504 as additives for use at 30 to 130 volume percent of abrasive grain to improve abrasive grinding wheel designed for lapping, polishing and grinding hypodermic needles.
The problems of the prior art are avoided to a significant degree by utilizing the abrasive tools and grinding process of the invention. In particular, the inventive formulation and process for manufacturing self-dressing grinding wheels permits the grinding of fine hollow metal tubes (cannula) to form the tapered tips on hypodermic needles without leaving burrs or metal fines on the needle tips.
The abrasive tools of the invention are "self-dressing" in the sense that the tool structure permits bond fracture to occur at an optimum rate, i.e., faster than prior art bonds, but not so fast as to shorten tool life beyond an economically acceptable amount. The tool structure allows coolant flow at the grinding face and removal of grinding debris before the debris loads the grinding face of the tool, necessitating dressing of the tool face. Depending upon the particular grinding process and the particular workpiece, dressing may be eliminated completely or the number of dressing cycles over the life of the wheel may be substantially reduced by utilizing the self-dressing tools of the invention which do not load with metal fines during the grinding operation.
The tools of the invention also cause less workpiece burn and draw less power during grinding than the tools used in the past because the grinding face of the tools of the invention tend not to load with metal fines.
While the tools of the invention are particularly useful in the needle tip grinding method of the invention, their bond system, grain content and balanced friability/strength functional properties make these abrasive tools suitable for similar grinding operations. such operations include the grinding of suture needle tips, lanceted surgical or medical devices or tools and other medical devices such as trocars. These devices and tools all have precise geometric shapes and rigid specifications for control of metal fines or burrs.